How many leads are you pushing to Facebook? Perhaps it just didn't make any matches?

Primarily it is looking up to match on email addresses, so if you're a B2B company you might find this functionality limited for Facebook.

You could try switching the matching function to 'Advanced matching' to see if you can pick up more.

Lastly, you might want to check if Facebook has any restrictions in your jurisdiction on the size of the custom audience. If it is a small number I imagine FB wouldn't bother with a list because people within might be individually identifiable.